The present invention relates to ceramic moldings small in thermal expansion coefficient, and, more particularly, to catalysts for purifying the combustion exhaust gases for use in vehicles, industries and for public welfare.
The cordierite series ceramics are mostly used as ceramics of low thermal expansion coefficient in the conventional catalyst carriers for purifying the exhaust gases. In addition, alumina series ceramics and mullite series ceramics are also used. However, these alumina series ceramics and mullite series ceramics have three to five times the thermal expansion coefficient of many cordierite series ceramics, thus resulting in lower practical rate.
The cordierite ceramics, having the formula 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, are composed of a mixture of talc (Mg.sub.3 (Si.sub.4 O.sub.10)(OH).sub.2), kaoli (Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4) and alumina (Al.sub.2 O.sub.3). The mixture is deaired, molded, dried, sintered.
The respective component % by weight of the theoretical composition cordierite is MgO 13.8%, Al.sub.2 O.sub.3 34.9%, SiO.sub.2 51.3%.
Also, the sintering temperature is about 1,400.degree. C. and the sintering is normally performed for about one week.
This thermal expansion coefficient is 1.2 to 1.4.times.10.sup.-6 /degree (room temperature .about.1000.degree. C.).
Even in the alumina series and mullite series ceramics, the sintering temperature is as high as 1,300.degree. C. or more, as disclosed in Japanese Patent Publications (examined) No. 54-1565 and No. 51-20358.
In this manner, in the conventional catalyst carrier composed of cordierite series ceramics, alumina series ceramics or mullite series ceramics, the high-temperature sintering operation was performed to produce substances small in thermal expansion coefficient so as to improve the mechanical strength. These carriers of the conventional method became extremely glassy during the sintering operation, thus considerably reducing the specific surface area i.e., the specific surface area in the case of the cordierite series ceramics was 1 m.sup.2 or lower per gram. Accordingly it is difficult to carry the catalyst substance directly on the conventional catalyst carrier.
Even if the catalyst carrying operation is performed, it is insufficient as the catalyst for purifying the exhaust gases. This is because the carried noble metal is likely to move and diffuse on the carrier during heating, thus suddenly deteriorating the catalyst performance.
Thus, in the conventional catalyst carrier composed of this type of cordierite series, alumina series or mullite series, the prior treatment, a so called wash coat, in which powders such as .gamma.-Al.sub.2 O.sub.3 or the like are coated on the surface is carried beforehand. Thereafter, the catalyst substance is carried.
Also, the catalyst carriers composed of the conventional cordierite series ceramics when sintered, contract by about 30% respectively in three dimensions during the scattering operation. Thus, they are difficult to maintain as to accuracy of size, are complicated to manufacture, and are inferior in product yield. Also, the cost thereof becomes higher because of the high sintering temperature. Furthermore, when the thermal expansion coefficient was 1.2 to 1.4.times.10.sup.-6 per degree (room temperature .about.1,000.degree. C.) cracks and splitting occurred during practical use. In particular, in the catalysts, for the vehicles, the thermal shock requirements have become stricter in recent years due to the higher performance of the engine so that thermal expansion coefficients are obliged to be reduced.